The present invention generally relates to binders used with sand casting processes and operations. More particularly, this invention relates to an improved binder for cores of the type which are placed in a mold cavity to form the interior surfaces of a casting, wherein the binder is characterized as imparting improved hot strength to the core while also being composed primarily of gelatin so as to be water soluble and nontoxic, and therefore capable of being readily and economically removed from the sand after the casting process, so as to facilitate the recycling of the sand for continued use without imposing an environmental hazard.
Metal casting is a widely practiced process for making ferrous and nonferrous articles which involves providing a mold cavity and pouring molten metal into the cavity so as to form a cast article. Where an intricate and complex casting is required, a core molding process is employed which entails placing a shaped and somewhat rigid form or core within the mold cavity to form particular features of the casting, such as interior surfaces of cavities and intricate features in the casting's exterior surface. One or more cores can be readily used in a casting process, either as assembled units or individually in separate areas of the mold cavity. Cores can be used in both sand-casting and permanent-mold casting methods and are typically formed of sand.
The shape of the core is maintained by the use of binders which adhere the sand particles together. The type of binder employed depends upon such factors as the green strength required of the core for handling, the anticipated interval between the time the core is formed and used, possible degradation by moisture or other atmospheric conditions, and the hot strength required of the core during the molding operation for maintenance of the core geometry until the casting has sufficiently solidified.
Hot strength is a particularly important property for cores used in squeeze casting processes, wherein high pressure is typically applied by a hydraulic press and maintained on the molten metal as it solidifies. Squeeze casting is a highly desirable technique for forming castings because it can be used for a variety of ferrous and nonferrous alloys to produce pore-free, fine-grain castings with excellent mechanical properties. Furthermore, squeeze casting is a relatively economical process and can be automated to operate at high rates of production. However, in combination with the elevated temperatures created by the molten metal, the high pressures associated with squeeze casting are particularly detrimental to the structural integrity of sand cores used within the mold cavity.
In the prior art, various types of binders are known, both organic and inorganic, for adhering the sand particles together. Linseed oil-based organic binders are widely used and contain a resin and thinner, such as high-grade kerosene, to provide good wetting and workability properties. Other known organic binders, including plastics of the urea- and phenol-formaldehyde groups, are also widely used. These organic binders generally entail a two-part polymer resin which is set during the core forming process so as to form an extremely durable core.
While plastic binders have generally performed well in iron casting operations, such binders are not readily volatilized or broken down at lower molding temperatures, such as those associated with aluminum casting operations. As a result, all or some of the core sand may not be readily removable from the casting after it has cooled. Obviously, a casting will be unusable if the core cannot be removed from the casting. Furthermore, where the core has not sufficiently degraded to allow the sand to flow freely from the casting, mechanical operations used to forcibly shake or extract the core from the casting may result in the destruction of the casting. In addition, because most current binders are thermoplastic materials, they also tend to flow or distort under the high heat and pressure of a squeeze casting operation, and therefore do not produce dimensionally accurate cavities when used in such operations.
An additional shortcoming of these conventional binders is that gases produced when such binders are volatilized are undesirable from the standpoint of causing porosity in the casting, as well as the potential environmental hazards which they may pose. Environmental hazards often persist after the casting operation when the core sand is disposed of or recycled for reuse within the foundry. Whether the binder residue is soluble in some medium or must be combusted, the process necessary to reclaim the sand will often have a harmful byproduct which cannot be returned to the environment because the binder residue within the byproduct may leach off into the ground water and cause contamination. Because such binders are not usually removable by use of a solvent, the need to burn the residue becomes a costly disadvantage which still poses an environmental hazard in the form of air pollution. Furthermore, disposal of the used core sands without clean-up is costly with current binders because the binder residues are toxic and render the sand/binder mixture a toxic waste, requiring disposal in an appropriate land fill.
As a result, binders which can be more readily extracted from the sand with less concern for their impact on the environment are generally preferred. Such binders include corn flour and dextrin, which both rely upon the hydrolysis of starch to form a colloidal product which can bind the sand particles together. However, both of these binders must often be used in conjunction with adjuncts, such as urea- or phenol-formaldehyde resins and acid catalysts, to achieve adequate green strength and/or improve its shelf life. As an example, U.S. Pat. No. 4,711,669 to Paul et al suggests mixing the reaction product of glyoxal, urea, formaldehyde, ethylene glycol, an acid catalyst and a solvent with a polyol to improve the crosslinking of the polymers and thereby improve the resistance of the core to deterioration by moisture. However, a particular disadvantage to increasing the crosslinkage of the polymers is that higher temperatures are necessary to break the bonds of the polymer structure, and therefore sufficiently degrade the binder so as to free the sand from the casting. In addition, such an adjunct-doped binder poses to some degree the same environmental hazards and economic disadvantages noted above with plastic binders.
Other alternatives known in the prior art include protein binders such as gelatin, casein and glues. These binders may generally be characterized as providing improved flowability of the sand, high binding power, rapid drying, fair resistance to moisture, and a low burn-out temperature, with only a small volume of gas being produced as the binder becomes volatilized. Protein binders also have a crystalline structure and therefore do not have a transition temperature for melting as do thermoplastics. As a result, protein binders do not soften when heated, but hold their crystalline structural shape until they begin to decompose by oxidation. In addition, disposal of protein binders does not pose an environmental hazard in that such binders readily decompose to a nontoxic form. However, the nature of the green strength of cores made with protein binders and their slow breakdown by oxidation generally hinder the ability of protein binders to be tailored to a particular casting process, in view of such factors as temperature, pressure and casting size.
Thus, it would be desirable to provide a core sand binder which is relatively economical to use and provides adequate structural strength to the core and, in particular, sufficient hot strength to withstand the high pressures associated with squeeze casting processes. In addition, it would be desirable that such a binder be readily and controllably degraded at elevated temperatures and easily washed from the sand, so as to permit the core sand to be recycled within a foundry operation or returned to the environment without posing an adverse environmental impact. Moreover, it would be particularly desirable if such a binder would have the above characteristics even when used with relatively low temperature casting operations, such as that associated with casting aluminum alloys.